1. Field of the Invention
The present invention generally relates to illumination subsystems of a metrology system, metrology systems, and methods for illuminating a specimen for metrology measurements. Certain embodiments relate to reducing the coherence of pulses of light for metrology applications.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. Metrology processes are used at various steps during a semiconductor manufacturing process to monitor and control the process. Metrology processes are used to measure one or more characteristics of the wafers. For example, metrology processes are used to measure one or more characteristics of a wafer such as a dimension (e.g., line width, thickness, etc) of features formed on the wafer during a process such that the performance of the process can be determined from the one or more characteristics. In addition, if the one or more characteristics of the wafer are unacceptable (e.g., out of a predetermined range for the characteristic(s)), the measurements of the one or more characteristics of the wafer may be used to alter one or more parameters of the process such that additional wafers manufactured by the process have acceptable characteristic(s).
In general, metrology processes can take a relatively long time, particularly when the number of sites on the wafers at which measurements are performed is relatively large. One obstacle to reducing the time in which metrology processes can be performed is the substantial difficulty of reducing the time involved in moving the wafer and/or metrology system optics such that multiple sites on the wafer can be measured. Therefore, one approach to decreasing the time involved in metrology processes involves continuously moving the wafer and/or metrology system optics relative to one another during the metrology process. However, such an approach significantly reduces the amount of time in which the measurement can be performed. Therefore, such an approach requires a light source that can produce a sufficient amount of light in a substantially short period of time. One such light source is a pulsed laser light source. Such light sources have a disadvantage in that the light has a speckle pattern due to the coherence of the light, which can interfere with the metrology measurements. As such, a significant obstacle to using such a light source in a metrology system that performs measurements as the wafer and/or metrology system optics are continuously moved relative to one another is that the speckle pattern must be suppressed relatively quickly, and particularly quicker than the time in which the speckle pattern can be suppressed using mechanical devices, time averaging procedures, or other currently used methods for suppressing speckle patterns.
Accordingly, it would be advantageous to develop illumination methods and/or subsystems for metrology systems that can provide adequate light with a sufficiently suppressed speckle pattern in a substantially short amount of time.